In the general field of disposable absorbent articles and structures, materials exhibiting specific fluid distribution properties are well known. Such materials became more and more relevant with the introduction of highly absorbent materials, also called Absorbent gelling materials or superabsorber, which do provide a good means for storing aqueous fluids such as urine, but do not enhance fluid transport, and even reduction of fluid transport can occur when sub-optimal designs and/or suboptimal materials are employed, and phenomena often referred to as "gel-blocking" take place. For example, in structures where the superabsorbent is homogeneously mixed with cellulose fibres, a certain critical concentration, which is strongly depending on the choice of the superabsorbent material, should not be exceeded in order to not deteriorate efficacy of the absorbent core.
As a consequence, a vast number of absorbent core designs have appeared with a separated functionality of storage and fluid distribution. This can be exemplified by EP 0 397 110 (Latimer) disclosing an absorbent article comprising a surge management portion for improved fluid handling, having specific basis weights, acquisition times and residual wetness; U.S. Pat. No. 4,898,642 of Moore et al. discloses specially twisted, chemically stiffened cellulosic fibres and absorbent structures made therefrom; EP0 640 330 (Bewick-Sonntag et al.) discloses the use of such fibres in a specific arrangement with specific superabsorbent materials. EP 0 397 110 (Latimer) discloses an absorbent article comprising a surge management region for improved fluid handling, having specific basis weights, acquisition times and residual wetness.
Initially, the requirements for a distribution material were not very high, and standard paper tissue materials such as used as wrapsheets for the cores and described for example in U.S. Pat. No. 3,952,745 (Duncan), were applied to also enhance the fluid distribution, as described in EP-0 343 941 (Reising) or U.S. Pat. No. 4,578,068 (Kramer).
As some of these materials did exhibit an undesired hard feel, methods for post formation treatments were well known to improve softness. "Post formation treatment" refers to the fact that--instead of, or, in addition to increasing softness during the making or formation of the tissue--the tissue is treated mechanically in a separate process step after forming and drying of the tissue, often just prior to further processing such as combining the tissue with other materials to form an absorbent core or article. Examples for such treatments are U.S. Pat. No. 5,117,540 (Walton) or U.S. Pat. No. 4,440,597 (Wells).
With the wish to improve the functionality of the absorbent articles, more specific requirements for distribution materials developed, such that porous materials were investigated in more depth. Examples are cellulosic foams such as commercially available by Spontex SA. France or High Internal Phase polymerized materials such as described in U.S. Pat. No. 5,268,224 (DesMarais). In order to improve the longitudinal fluid distribution, high surface area synthetic fibres were applied in absorbent structures, such as described in US Statutory Invention Registration H1511.
These approaches have in common, that they are complex to manufacture and hence relatively expensive when compared to cellulosic fibre-based materials.
Hence, early approaches aimed at improving the wicking properties of cellulose fibre-based materials, such as U.S. Pat. No. 3,575,174 or U.S. Pat. No. 4,781,710, whereby parts of the structure are compressed to a higher density, thus creating smaller pores for increased wicking height, for example, along "wicking lines" or in closed mesh patterns. Such attempts also aimed at achieving a certain preferential fluid distribution direction. However, in these approaches the size of the large pores was reduced relatively more than of the smaller pores, such that the positive impact of the increased wicking height was counterbalanced by reduced amount of fluid which could be transported to such heights.
Other attempts to impact on the pore size of distribution materials is described in U.S. Pat. No. 5,244,482 (Hassenboehler), aiming at reducing maximum pore size by stretching a fibrous structure comprising meltable fibres in one direction and "freezing" the deformation by heat curing.
Also, special material composites were developed, aiming at allowing to tailor the pore size and pore size distribution. Examples for such improvements are described in greater detail in commonly-assigned, co-pending (allowed) U.S. patent application Ser. No. 08/382,817, filed Feb. 3, 1995 in the names of Horney et al., entitled "Fluid Distribution Member for Absorbent Articles Exhibiting High Suction and High Capacity", or in commonly-assigned, co-pending U.S. patent application Ser. No. 08/633,630, filed Apr. 17, 1996 in the names of Seger et al., entitled "High Capacity Fluid Absorbent Members". Both aim essentially at providing a resilient structure by using specially stiffened cellulosic fibres such as crosslinked cellulose soft-wood fibres, and by filling the large pores with small and thin cellulosic fibres such as eucalyptus fibres. Both applications further add means for providing sufficient integrity and strength to the structure, the first one (U.S. Ser. No. 08/382,817) by adding thermoplastic fibres and partially melting these, the second (U.S. Ser. No. 08/633,630) by adding a chemical binder.
However, it has been realised, that in spite of more complex technologies, all modifications are still limited by the inverse relationship of wicking height and wicking flux, i.e., wicking height can be increased, but at cost of reduced wicking flux. There has been no recognition of the ability to improve both at the same time.
Surprisingly, the inventors have been able to overcome this incompatibility by carefully selecting the criteria to assess materials and thus identifying the appropriate materials to satisfy these characteristics.
Hence the present invention is concerned with providing these selection criteria to be able to identify materials which both improved wicking height and flux.
A further object of the present invention are materials with both increased wicking height and flux at the same time.
A further object of the invention are materials with improved directionality of transport properties, especially in the length (often also called machine) direction of the web.
Still a further object of the present invention is to provide absorbent articles comprising such materials.